The process of expansion cooling was first devised by Siemens in 1857. The process involves precooling a mechanically pumped high pressure gas and then expanding the highly compressed gas through a small orifice and into a cooling vessel. The expanding gas falls in temperature because of doing external work against the pressure in the cooling vessel, also known as the Joule-Thompson effect. Continued operation results in the liquefaction of the gas, which liquid accumulates in the cooling vessel and is available for cooling of an external object.
Cryogenic refrigeration systems are commonly used in the aerospace industry to cool devices such as infrared sensors. It can readily be appreciated that mechanical compressors are troublesome for such applications, not only because of service difficulty, but also particularly because of the introduction of undesirable vibration.
Cryogenic refrigeration systems based on physisorption or chemisorption compressors were developed in response to these needs. Although the principles of both physical and chemical sorption have been studied for well over a century, it was not until 1963 that Vickers of JPL proposed using sorption compressors for a Joule-Thompson refrigeration. Vickers' process is set forth in U.S. Pat. No. 3,270,512.
The so-called sorption compressors operate on the basis of reversible sorption of a gas into or onto a suitable sorbent. In general, a particular gas may be physically adsorbed or chemically absorbed by the sorbent at relatively low pressure and temperature. Heating the sorbent causes the gas to be desorbed at high pressure. The high temperature, high pressure gas may then be precooled and passed through a Joule-Thomson valve where it is expanded and partially liquefied to provide cooling.
The prior art has shown that certain benefits result from the use of multicomponent gas refrigerants. For example, British Patent No. 1,336,892, issued to Alseev et al. on Nov. 14, 1973, discloses the use of such multicomponent gas refrigerants. It was shown by Alseev et al. that the use of multicomponent gas refrigerants could increase cooling efficiency by as much as 10 to 12 times that of the known prior systems.
The known cryogenic refrigerator systems based on sorption pumps have operated with only a single gas. The aerospace industry has been unable to effectively utilize the multigas technology in sorption based systems because such multigas refrigerants have, prior to the present invention, required the use of mechanical compressors. An example of a single gas sorption refrigerator is disclosed in U.S. Pat. No. 4,697,425 issued to Jack Jones on Oct. 6, 1987.
In order to obtain temperatures as low as 80K with such single-gas sorption systems, it has been necessary to use separate thermally coupled sorption stages. For example, cooling to 80K has been achieved, but only by using a charcoal krypton physisorption stage (140K) in combination with an 80K oxygen chemisorption stage. Because of thermal losses at the stage interfaces and other effects, such systems typically require about 180 watts of power per 1 watt of cooling at 80K. It is possible to substantially reduce the power requirement, but only at the cost of increased system complexity.